Recent improvements in patient survival has resulted in widespread use of liver transplantation as therapy for end stage liver disease. The pathologist plays a critical role in the pre- and post-operative care of these patients, and the core needle biopsy of the allograft has become a fairly reliable method of diagnosing acute cellular rejection. Most of the non-rejection related causes of graft dysfunction produce morphologic manifestations similar to those seen in nontransplanted livers (e.g., duct obstruction resembles duct obstruction). Many pathologists are, however, unfamiliar with the histologic changes of the various types of rejection. The following article is an attempt to acquaint pathologists with the morphologic features of liver allograft rejection. As a backdrop to understanding the events in humans, observations in untreated experimental animals are presented and reviewed briefly.